A study of ultrasonic bonding of minute Au bumps

We studied on ultrasonic bonding of Au bumps. Recently some commercial bonding machines that are furnished ultrasonic vibrator have been developed. In these machines bonding temperature can be decreased drastically. Many Au bumps (width 50 mum) were made on Al pads using photo-lithography technology. Three kinds of UBM (under bump metal) that Ti evaporation, Cr evaporation, and the non-electrolyzed plating nickel were selected. And moreover a sample without UBM was produced, too. We bonded these samples each other using the ultrasonic bonding machine (frequency 50kHz). Bonding temperature was 150degC. Generally when bonded without ultrasonic machine, bonding temperature is over 350 degC. After bonding, die shear tests were conducted. In these results, bonding load was the higher, shear strength was the higher. These shear strength levels depend on the kinds of UBM. Attractive result was obtained, that the sample without UBM had the highest shear strength (over 100MPa) in any bonding loads. Furthermore, fully being bonded was confirmed by observation of the fracture surface after die shear examination by SEM. And the cross sections of bump samples were observed using FIB, SEM and EBSP. We found out that some bands of ultra-fine grains exist in the Au bumps. Its position was about half of its bump height. Moreover, we challenged to the finer bump size (10 mum width). All ten thousand bumps were successfully bonded by optimizing the bonding parameters